Heat exchangers including flat tubes and fins have been known in the art. For example, in a heat exchanger described in Patent Document 1, laterally extending flat tubes are arranged to be spaced from one another in the vertical direction (i.e., the upward and downward directions) by a predetermined distance, and plate-like fins are arranged to be spaced from one another by a predetermined distance in the direction in which the flat tubes extend. In each of heat exchangers described in Patent Documents 2 and 3, laterally extending flat tubes are arranged to be spaced from one another in the vertical direction by a predetermined distance, and a corrugated fin is provided between each adjacent ones of the flat tubes. In these heat exchangers, air flowing while being in contact with the fins exchanges heat with fluid flowing in the flat tubes. As illustrated in FIG. 2 of Patent Document 2 and FIG. 4 of Patent Document 3, fins in heat exchangers of this type are provided with louvers for promoting heat transfer.